In typical electrophotographic copier machines, a latent image is produced on a photoconductive surface; toner, typically a black substance, is spread onto the latent image and is attracted to that part of the image which carries a charge, e.g., an image of typewritten lines while being unattracted to discharged areas, e.g., the white sheet of paper. A copy sheet (in plain paper copiers) is then placed in juxtaposition with the toner image and receives a transfer of toner which is then heated and bonded to the copy paper forming the finished copy.
In the above machine, toner is brought to the latent image at the development zone by various methods, including the deposit of toner on small carrier beads made of magnetic material. Toner is attracted to the carrier beads by a triboelectric effect between a thin polymeric coating on the magnetic beads and the toner itself. When the carrier is brought to the development zone, the beads are crushed together with a jarring effect sufficient to overcome the triboelectric effect and release the toner for attraction to the charged part of the latent image.
In magnetic brush developer units wherein the carrier particles are moved from a pickup zone near the bottom of the rotation of the magnetic brush roll and moved upwardly to a development zone, it is necessary to provide the surface of the magnetic brush roll with a sufficiently roughened exterior in order to mechanically seat the spherically-shaped carrier beads as the magnetic forces produced by the magnetic brush roll hold the beads to the exterior surface while the beads are carried upwardly. If the exterior surface of the roll does not provide a sufficient seating surface for holding the beads in place, poor copy quality will result from insufficient developer flow, i.e., spotty copy.
On the other hand, if the exterior surface of the roll is inordinately rough, the spherically-shaped carrier beads will be too tightly mechanically locked in place and will not be able to tumble and roll on one another in the development zone where they are brought into contact with the latent image. Such a condition will also provide poor copy quality and has the further detrimental effect of wearing the surface of the carrier when the carrier beads are crushed together without the ability to tumble and move. Since the surface of the carrier is coated with a polymeric material to provide a triboelectric effect between the carrier and the toner, the polymeric coating is degraded by such action. A further detrimental effect occurs in that the toner material is ground against the surface of the magnetic brush roll where, after a period of use, it forms a toner film along the surface of the roll. This condition is especially true of toners with low temperature melting points of the types required with a hot roll fuser. The result is an erratic and unpredictable magnetic brush roll bias voltage which should be stable for minimal copy background and for copy density control.
It is, therefore, an object of this invention to provide a magnetic brush developer roll surface which provides an adequate mechanical interlock with the carrier beads to move the beads with adequate flow to the development area.
It is a further object of this invention to provide a surface for a magnetic brush roll which allows adequate tumbling and movement of the beads relative to each other and to the roll surface when the development zone is reached.